A thick layer of toxic foam has covered the surface of São Paulo's largest and most important Tiete River.
Tiete River, with a length of 1150 kilometres and a basin area of 150 thousand square kilometres, runs along the coast through São Paulo and dozens of smaller municipalities, feeding an important hydroelectric power plant that supplies energy to a large part of the region.
Untreated domestic runoff from 39 cities in the metropolitan area is killing aquatic river life as well as polluting riverbanks and emitting noxious odours in riverside communities.
The river has been heavily polluted for decades, and government efforts to clean up the river have been insufficient